Method for producing non-ageing cold rolled steel sheets having good press-formability by continuous annealing

ABSTRACT

A method for producing a cold rolled steel sheet (strip) having both non-ageing property and excellent press formability by continuous annealing.

United States Patent [191 Condo et al.

[451 Apr. 22, 1975 22 Filed: Nov. 15, 1973 211 App]. No.: 416,059

[30] Foreign Application Priority Data [51] Int. Cl C2ld 9/48 [58] Field of Search 148/12 C [56] References Cited UNITED STATES PATENTS 3.239.390 3/1966 Matsukura et a1. 148/12 C 3.248.270 4/1966 Laidman et a1 148/12 C 3.492.173 1/1970 Goodenow 148/12 C 3.806.376 4/1974 Toda et al 148/12 C Primary E.\'aminer-W. Stallard Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] ABSTRACT A method for producing a cold rolled steel sheet (strip) having both non-ageing property and excellent Nov. 20, 1972 Japan 47-115649 press f bim by ontinuous annealing.

52 us. Cl 148/12 c 10 Claims 1 Drawing Finishing Temperature X 600-650C (excluded) A 650700'C (excluded) 0 voo-aso'c (included) 880-960'C (included) D 980*1000'C Appropriate Coiling Temperature LA Z5 I 1 l o I I 0 Appropriate Finishing A Temperature Range 20- T l v I O v l u l o 1.5 l X l n o I l l l 10 I l l (b) Strain Ageing 3 l l v f Index /mm) 1 I I l l v 1 Coiling Temperature ('C) PATENTEBAFRZZIBYS 3,879,232

Finishing Temperature Appropriate Coiling Temperature Range I O I 0 Appropriate Finishing I A Temperature Range I I v OI I I I x X r: 1 1.5 I U o I I I I x I I 10 I I n I i I I (b) Strain Ageing 3 I I Index /mm) 2 I I I I 1 O 260 1.00 soo soo Coiling Temperature ('C) METHOD FOR PRODUCING NON-AGEING COLD ROLLED STEEL SHEETS HAVING GOOD PRESS-FORMABILITY BY CONTINUOUS ANNEALING The present invention relates to a method for producing a cold rolled steel sheet (strip) having both nonageing property and excellent press formability by continuous annealing.

Press forming grades of cold rolled steel sheets. for applications such as auto bodies are required to have excellent deep drawability and stretchability and further good ageing property.

In a deep drawing press-forming operation. only a small force for preventing wrinkles is given to the steel sheet. the sheet being formed by a punch pressing the steel into a die. Therefore. the most important property required for press forming steel sheet is a high Tvalue. On the other hand. during a stretching operation. the force preventing wrinkles is remarkably increased. and only a slight portion of the steel sheets are pressed into the dies and only portions contacting the punch are expanded and pressed. In this case. the most important characteristic is a high Erichsen value. and additionally it is desired that the yield stress is low both for dee drawability and stretchability. and that elongation is large.

Cold rolled steel sheets for subjection to press forming are generally annealed after cold rolling. but when usually subjected to temper rolling to reduce the yield point elongation. Even in this case. however. if the sheets are left for a long time before press forming. the yield point elongation is gradually restored to its former high value. due to the ageing of carbon and nitrogen in solid solution. and thus stretcher strains are again caused during press forming.

Cold rolled steel sheet in which the yield point elongation does not return to its original value. even when the sheet is left for a long time after temper rolling. is called non-ageing steel sheet. and higher press-forming grades of cold rolled steel sheet must be non-ageing.

Cold rolled non-ageing steel sheets having deep drawability and stretchability for automobile bodies have been conventionally produced on commercial scale by box annealing an Al-killed steel in sheet form. However. box annealing requires normally more than 60 hours to complete. even in case when low grades of cold rolled steel sheets which cannot be press-formed are treated. and in case of higher grades of cold rolled steel sheet for press-forming of automobile bodies. slow heating is necessary in order to improve 'Fvalue through precipitation of AlN during annealing. thus further elongating the treating time and lowering the production efficiency. I

Therefore. it is a primary object of the present invention to provide a method for producing high grades of cold rolled steel sheet suitable for press forming of automobile bodies using a continuous annealing system which give high production efficiency.

ln conventionally known arts. a large amount of Al addition is required and a high temperature coiling operation is required after hot rolling.

According to the present invention. non-ageing property of the steel can be obtained even when Al addition is small. the coiling temperature after hot coiling is low. and descaling is easier.

The method of the present invention is characterized in that a steel containing not more than 0.015 percent of carbon. and containing manganese. aluminium. sulfur. oxygen and nitrogen so as to satisfy the following condition:

is hot rolled with a finishing temperature between 650 and 980C. at coiling temperature between 300 and 600C. then cold rolled. and thus obtained cold rolled steel sheet is subjected to continuous annealing.

The steel contains impurities such as S. O. N etc.. and it is natural that the steel shows better press-formability when these impurities are smaller. However. it is not desirable to reduce these impurities to extremely low contents in a commercial production because it increases production cost remarkably.

The basic inventive idea of the present invention is to render the impurities as harmless as possible and to utilize these impurities for improvement of the steel.

Molten steel obtained by an ordinary steel making method such as by a convertor contains about 500 ppm of oxygen. When 0.5 2.0 kg of Al per ton of molten steel is added before ingot making or before continuous casting. A1 0 is formed. which is removed. Thus the oxygen content can be reduced to about 50 ppm. By fixing the residual oxygen as A1 0 by Al. it can be possible to improve the quality of the steel. Therefore. it is necessary that Al is retained in an amount in correspondence to the amount of the residual oxygen. On the other hand. Al fixes N as AlN at low temperatures after the ingot solidification. thus improving the steel quality. Therefore it is necessary that Al is retained additionally in an amount corresponding to the amount of nitrogen.

Therefore. the steel ingot to which the present invention is applied must contain Al in an amount determined by the following formula 2Xlatomic weight of Al) 3 (atomic weight of 0) atomic weight of Al AHr/l) atomic weight of N 2 by vacuum degassing etc. in order to improve the 7 value.

Also. in order to improve theTvalue and at the same time to obtain non-ageing property of the steel. it is desirable that the carbon content is lowered to 0.008 per cent or less.

According to the present invention Al may be added to the steel during the last stage of the ingot making or may be added dividedly before the ingot making and during the last stage of the ingot making, whereby a steel ingot having a rim layer or a similar layer in the surfacial thin portion. and a core portion of the composition defined by the present invention is obtained. and when this steel ingot is processed into a cold rolled steel sheet. the surface of the cold rolled steel sheet is covered by the rim layer which contains substantially no aluminium. and thus a cold rolled steel sheet having excellent coating adhesion for zinc-plating can be obtained.

The process of fixing the oxygen as Al- O by Al in completed at the solidification of the molten steel if the steel composition is adjusted as defined in the present invention. but in order to fix nitrogen as AlN by Al. special operations are required during the hot rolling and the continuous annealing which are inventive features of the present invention just as the above defined steel composition.

Thus. the present invention is completely different from the conventional method in which the combination of AlN is attained by a high temperature coiling after the hot rolling.

According to the present invention. a special operation is conducted so as to provide a precipitation site of AlN during the hot rolling. Namely. the hot rolling finishing temperature is defined between 650 and 980C. the steel is rapidly cooled to the coiling temperature. and coiled at a temperature between 300 and 600C. The above finishing temperature range includes A transformation point. and when the finishing is done above the A, transformation point. the steel is cooled rapidly through the transformation point to increase the intergranular area so as to provide the precipitation site of AlN. and when the finishing is done below the A;, transformation point. the dislocation increased by the rolling is partially retained by the subsequent rapid cooling so as to provide the precipitation site of AlN.

When the finishing temperature is higher than 980C. grain growth takes place during the cooling just above the transformation point. so that it is impossible to increase the intergranular area even after the passthrough of A transformation point. On the other hand if the finishing temperature is lower than 650C the retained dislocation is excessive so that AlN becomes too fine during the subsequent steps and falls outside the appropriate range of the present invention.

When the coiling temperature is higher than 600C. the restoration of the dislocation is effected during the slow cooling after the coiling operation. or AlN precipitates too coarsely before the present inventive features are applied in the subsequent steps. so that the desired results of the present invention cannot be obtained.

On the other hand. when the coiling temperature is below 300C. the diffusion rate of Al and N is slow so that it is impossible to form Al-N cluster which precipitates AlN in the subsequent step.

Therefore. according to the present invention. the hot rolling finishing temperature range is defined from 650 to 980C. and the hot coiling temperature range is defined from 300 to 600C and the fine Al-N cluster which is a pre-stage of the AlN precipitate is densely distributed.

The most desirable conditions are the hot rolling finishing temperature range from 700 to 880C; the cooling rate from l0 to l00C/second down to the hot coiling temperature. and the hot coiling temperature range from 400 to 550C.

If the cooling rate after the hot rolling finishing is below 10C/second. the effect of increasing the intergranular area during the passage through the A transformation point is weakened, and the restoration of the discolation formed during the rolling progresses to some degrees and the results of the present invention are less remarkable. On the other hand. when the cooling rate is higher than l00C/second. the cooling is too rapid to obtain uniform cooling for a commercial production and the shape quality of the steel sheet is some times deteriorated.

In case of the hot rolled steel sheet in which the fine Al-N cluster is densely formed during the hot rolling. AlN can be easily and densely formed even by continuous annealing in which short time heating and soaking are applied to the cold rolled steel sheet. and nitrogen can be fixed as AlN completely by Al. Also there are many lattice defects at the intersurface between AlN and the matrix which serve as the precipitation site for carbon. and AlN having such intersurface is densely distributed so that the precipitation of carbon during the rapid cooling after the soaking or during overageing can be completed in a very short time. It is most desirable that tensile stress or repeated bending stress or both which gives l to 3 percent permanent strain is given to the steel sheet during heating and soaking in the continuous annealing. and that the heating rate from 300C to a temperature higher than the recrystallization temperature is maintained between 3 and 10C/second. These measures can be done in single or in combination. and are most efficient conditions for forming the AlN precipitate from the MN cluster.

The present invention will be more clear from the following examples described by reference to the attached drawings.

FIG. 1 (a) and (b) are respectively a graph showing the r value and the strain ageing index of the products obtained according to Example 2.

EXAMPLE 1 Steel ingot prepared by steel making in a convertor and vacuum degassing and having the composition shown in Table l was hot rolled with a finishing temperature of 780C. an average cooling rate of 23C/second to a coiling temperature and a hot coiling temperature of 450C. acid-pickled. and cold rolled with a reduction of 80 percent to obtain 0.8mm thick cold rolled steel sheets. The steel sheet thus obtained was subjected to recrystallization annealing at 700C for 1 minute and over-ageing at 350C for 3 minutes in a continuous annealing furnace. while repeated bending by a hearth, roll and 2.5- percent. p erma nent'strain' 6 EXAMPLE 2 Steel ingot of the same compositions as the specimen B-Z m Table- 1 was hot rolled with a finishing temperacomparison. the specimen (-1 -was not \"acuum- 5 ture between 600 and 1.000C, a hot coiling temperadegassed. and subjected to a box annealing at 700C for lurebetwefin 0 and 800C and then treated in a sim- 4 hours dh percgmliempgr lli i A i il ilar way as in Example 1. The r value and strain ageing way. The mechanical properties of these products are indfiX OfthUS Obtained pro cts a ho n IG- Shown i T bl 2, I and (1)). It is clear that when with the finishing tempera- The specimensA-Z A- 7 which are within the scope In ture between 650 and 980C and the coiling temperaof the present invention show higher yield stress. elonture between 300 and 600C. the T value is high and gation. Erichsen value and Tvalue than the specimens the strain ageing index is small. This means that both B-l C-1 and show smaller yield point elongation the press-formability and the ageing property are excelstrain ageing index after the ageing. and thus it is clear lent.

Table l Compositions of Specimens Composition (7r) 32 27 27x2 No. C Mn S O N A1 S (70) N W (9r) Remarks A-l 0.013 0.29 0.01 1 0.038 0.0041 0.008 0.0064 0.045 Out side present invention (Excessive 0) A-2 0.009 0.20 0.014 0.012 0.0038 0.017 0.0081 0.015 Inventive (Al added prior to ingot making) A-3 0.012 0.21 0.009 0.008 0.0042 0.019 0.0052 0.017 Inventive (Al added at last stage of ingot making) A-4 0.014 0.10 0.012 0.004 0.0035 0.020 0.0069 0.01 1 Inventive (A1 added prior to ingot making) A5 0.01 1 0.18 0.012 0.003 0.0029 0.029 0.0069 0.009 Inventive (AI added at last stage of ingot making) A-6 0.009 0.1 I 0.007 0.003 0.0046 0.041 0.0040 0.012 Inventiye (continuous casting A-7 0.009 0.17 0.008 0.004 0.0031 0.055 0.0046 0.008 Inventive (Al added at last stage of ingot making) A-8 0.014 0.20 0.009 0.004 0.0042 0.078 0.0052 0.013 Out side present invention (Excessive A1) 134 0.006 0.17 0.007 0.003 0.0039 0.036 0.0040 0.01 1 Inventive (vacuum degassed) B-2 0.003 0.09 0.007 0.003 0.0042 0.010 0.0040 0.01 1 C-1 004 0.31 0.012 0.004 0.0043 0.042 0.0069 0.016 Comparative Table 2 Mechanical Properties No. Yield Tensile Elonga- Erichsen Yield point elon- Strain point Strength tion Value T gation after ageing ageing Remarks g/ g/ (7%) (mm) at 100C for 60 min. index 14-1 19.2 32.7 44.3 10.3 1.22 .2 3.5 Outside Present Invention A-2 18.8 32.0 47.5 11.6 1.82 0.4 1.1 Inventive A-3 16.3 32.2 48.0 11.7 1.86 0.3 0.7 A-4 16.1 31.7 48.5 11.9 2.01 0.2 0.5 A-S 17.3 31.9 47.8 11.6 1.82 0.2 0.6 A-6 16.2 31.6 49.1 12.0 2.20 0.4 0.4 A-7 17.3 31.4 47.9 11.8 1.82 0.3 0.5 A-8 18.8 32.3 46.1 11.4 1.68 0.2 0.5 Outside Present Invention 8-1 15.0 31.2 49.8 12.1 2.38 0.0 0.2 Inventive 13-2 13.7 30.6 51.0 12.3 2.45 0.0 0.1 C-1 19.8 32.3 46.3 11.2 1.68 0.6 1.3 Comparative that the steel sheet according to the present invention is a non-ageing steel sheet having excellent press-formability. Also the strain ageing index of the present in: ventive steel sheet is remarkably smaller than that of the prior art (Japanese Patent Publication No. Sho 47-33409).

What is claimed is: l. A method for producing a cold rolled steel sheet 5 having non-ageing properties and press-formability comprising hot rolling a steel containing not more than 0.015% of carbon. and containing manganese. aluminum. sulfur. oxygen and nitrogen and wherein with a finishing temperature between 650 and 980C. coiling at a temperature between 300 and 600C. cold rolling and then continuously annealing the cold rolled steel sheet.

2. The method of claim 1 wherein the percentage of Mn is not more than (H5.

3. The method of claim 1 wherein the percentage of Al is not more than 0.06.

4. The method of claim 1 wherein the percentage of Mn is not more than 0.15 and the percentage of Al is not more than 0.06.

5. The method of claim 4 wherein the amount of carbon is not more than 0.008 percent.

6. The method of claim 1 wherein the finishing temperature is between 700 to 880C.

7. The method of claim I wherein the coiling temperature is between 400 to 550C.

8. The method of claim 1 wherein after hot rolling. the hot rolled sheet is cooled to the coiling temperature at a rate between 10 and C/sec.

9. The method of claim 1 wherein during the continuous annealing. the sheet is subjected to tensile or bending stress to produce 1 to 3 percent permanent strain in the sheet.

10. The method of claim 1 wherein in the continuous annealing. the sheet is heated from 300C to a temperature higher than its recrystallization temperature at a heating rate between 3 and l0C/second. 

1. A method for producing a cold rolled steel sheet having non-ageing properties and press-formability comprising hot rolling a steel containing not more than 0.015% of carbon, and containing manganese, aluminum, sulfur, oxygen and nitrogen and wherein
 1. A METHOD FOR PRODUCING A COLD ROLLED STEEL SHEET HAVING NON-AGEING PROPERTIES AND PRESS-FORMABILITY COMPRISING HOT ROLLING A STEEL CONTAINING NOT MORE THAN 0.015% OF CARBON, AND CONTAINING MANGANESE, ALUMINUM, SULFUR, OXYGEN AND NITROGEN AND WHEREIN
 2. The method of claim 1 wherein the percentage of Mn is not more than 0.15.
 3. The method of claim 1 wherein the percentage of Al is not more than 0.06.
 4. The method of claim 1 wherein the percentage of Mn is not more than 0.15 and the percentage of Al is not more than 0.06.
 5. The method of claim 4 wherein the amount of carbon is not more than 0.008 percent.
 6. The method of claim 1 wherein the finishing temperature is between 700* to 880*C.
 7. The method of claim 1 wherein the coiling temperature is between 400* to 550* C.
 8. The method of claim 1 wherein after hot rolling, the hot rolled sheet is cooled to the coiling temperature at a rate between 10* and 100*C/sec.
 9. The method of claim 1 wherein during the continuous annealing, the sheet is subjected to tensile or bending stress to produce 1 to 3 percent permanent strain in the sheet. 